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Rheology of Solder Paste: Shelf Life Study

Rheology of Solder Paste: Shelf Life Study
This study offers a better understanding of the correlation between rheological properties and shelf life of solder paste exposed to extreme conditions.
Materials Tech


Authored By:

Fan Gao Ph.D., Dinesh Amin
Kester Inc.


Can expired solder paste be used without an issue? Solder paste consists of powder and flux components that start chemical reactions immediately after blending. Therefore, during manufacturing, packing, transportation and storage, solder paste is continuously changing over time. Additionally, during usage varying temperature and humidity conditions can influence the reaction rate. Flux formulation is the key factor affecting the reaction between solder powders and flux, which will further impact the solder paste shelf life. Besides solder ball, slump and viscosity measurement, rheological analysis is one of the best methods to quantify the change of solder paste. In this paper, an accelerated shelf life prediction method is introduced to study the status of solder paste in a very short period. Several rheology test methods are developed to investigate the change of solder paste from different perspectives, including yield stress, shear sweep, strain sweep (oscillation mode) and axial force. Each rheological experiment has its limitation, and the responses are influenced by the test method used.

This paper examines the accuracy and precision of rheological experiments through four different solder pastes. Both no-clean and water-soluble type pastes are included to illustrate the environmental contribution to the paste degradation. This study forms the background for a better understanding of the correlation between rheological properties and shelf life of solder paste exposed to extreme conditions.


The accelerated solder paste aging method has been used to prepare paste samples and simulate the solder paste stored at high temperature environment. The reaction between flux and solder powder surface can be confirmed from the SEM imaging of dry paste. Several rheological test methods have been developed in this study to predict the shelf life of solder paste from the rheological measurements. In general, NC P1 performed the shortest shelf life among all tested samples from all test methods, including yield stress, shear sweep, and oscillation strain sweep methods, after aging at 40 ˚C for 3 days. The DOE analysis shows the formulation effect on the paste flux shelf life is most significant, and the storage temperature will increase the paste viscosity more than the storage time.

Since NC P1 has 1-year shelf life in the refrigerator, all other samples will have longer shelf life. NC P2 has approved 1-year shelf life at room temperature, which shows similar rheological results as WS P1. Therefore, it is predicted that WS P1 has at least 1-year shelf life at room temperature. In summary, proper chemicals formulation can minimize the reaction between flux and solder powder and therefore provide very long shelf life solder paste, for both no clean and water-soluble type solder pastes, even storage at elevated temperature at 40 ˚C.

Two methods under flow mode — yield stress and shear sweep shows more sensitive responses than oscillation mode strain sweep method, which is possibly due to the less breakdown structure under oscillation mode so not much flux-powder interaction can be measured. Temperature sweep test under oscillation mode shows both the viscosity and modulus is decreasing when temperature increases, which also may be used to predict the shear viscosity. The axial force test can be used to correlate with tackiness value measured by tackiness tester.

The study in this work established the repeatable methods to understand flux and paste rheological behavior, which provide useful tools for further understanding many other aspects of the paste formulation contribution to shelf life. It also builds the fundamentals to develop a repeatable and reproducible standard method to be applied in the electronic assembly industry for quality control.

Initially Published in the SMTA Proceedings


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